readahead: clean up the code, update docs

Remove dubious comments and #if 0 section.  Document new event counters,
and add one for read failures.

Signed-off-by: Zygo Blaxell <bees@furryterror.org>

diff --git a/docs/event-counters.md b/docs/event-counters.md
index d376a3da9e86b6ef7f41c1a2aa608f2fce22d688..237826df755a42a90fb2a284f77a6a48ad0ceffd 100644 (file)
--- a/docs/event-counters.md
+++ b/docs/event-counters.md
@@ -281,11 +281,14 @@ The `progress` event group consists of events related to progress estimation.
 readahead
 ---------
 
-The `readahead` event group consists of events related to calls to `posix_fadvise`.
+The `readahead` event group consists of events related to data prefetching (formerly calls to `posix_fadvise` or `readahead`, but now emulated in userspace).
 
+ * `readahead_bytes`: Number of bytes prefetched.
+ * `readahead_count`: Number of read calls.
  * `readahead_clear`: Number of times the duplicate read cache was cleared.
- * `readahead_skip`: Number of times a duplicate read was identified in the cache and skipped.
+ * `readahead_fail`: Number of read errors during prefetch.
  * `readahead_ms`: Total time spent emulating readahead in user-space (kernel readahead is not measured).
+ * `readahead_skip`: Number of times a duplicate read was identified in the cache and skipped.
  * `readahead_unread_ms`: Total time spent running `posix_fadvise(..., POSIX_FADV_DONTNEED)`.
 
 replacedst

diff --git a/src/bees.cc b/src/bees.cc
index 95e2b47eb66e8b8e4d4bfa09ecf20be62ac944f2..e4a96abf445e456c2ea85f7944fdede271efd615 100644 (file)
--- a/src/bees.cc
+++ b/src/bees.cc
@@ -246,10 +246,6 @@ bees_readahead_nolock(int const fd, const off_t offset, const size_t size)
        Timer readahead_timer;
        BEESNOTE("readahead " << name_fd(fd) << " offset " << to_hex(offset) << " len " << pretty(size));
        BEESTOOLONG("readahead " << name_fd(fd) << " offset " << to_hex(offset) << " len " << pretty(size));
-#if 0
-       // In the kernel, readahead() is identical to posix_fadvise(..., POSIX_FADV_DONTNEED)
-       DIE_IF_NON_ZERO(readahead(fd, offset, size));
-#else
        // Make sure this data is in page cache by brute force
        // The btrfs kernel code does readahead with lower ioprio
        // and might discard the readahead request entirely.
@@ -263,13 +259,16 @@ bees_readahead_nolock(int const fd, const off_t offset, const size_t size)
                // Ignore errors and short reads.  It turns out our size
                // parameter isn't all that accurate, so we can't use
                // the pread_or_die template.
-               (void)!pread(fd, dummy, this_read_size, working_offset);
-               BEESCOUNT(readahead_count);
-               BEESCOUNTADD(readahead_bytes, this_read_size);
+               const auto pr_rv = pread(fd, dummy, this_read_size, working_offset);
+               if (pr_rv >= 0) {
+                       BEESCOUNT(readahead_count);
+                       BEESCOUNTADD(readahead_bytes, pr_rv);
+               } else {
+                       BEESCOUNT(readahead_fail);
+               }
                working_offset += this_read_size;
                working_size -= this_read_size;
        }
-#endif
        BEESCOUNTADD(readahead_ms, readahead_timer.age() * 1000);
 }